This invention relates generally to air/fuel mixture controllers for large internal combustion engines used in industrial applications, such as for power generation and natural gas compression. More particularly, the invention relates to engines of this type that employ gaseous fuel and a carburetor to mix the fuel and air. Recently imposed emission control regulations will require operators of large industrial engines to reduce pollution rates by as much as 90% below current levels. Users of four-stroke engines fueled by natural gas are allowed to treat exhaust gases chemically in a catalytic converter, which converts offending pollutants, such as oxides of nitrogen (NO.sub.x), carbon monoxide (CO), and non-methane hydrocarbons, to much less harmful substances: nitrogen, carbon dioxide (CO.sub.2) and water (H.sub.2 O). Unfortunately, converter technology is not always sufficient to effect this conversion, unless the chemical balance of offensive pollutants entering the catalytic converter is maintained at a precise ratio. Maintaining the balance of pollutants entering the converter is effected by controlling the ratio of air to fuel input to the combustion engine.
Typically, the air/fuel ratio is controlled in response to measurement of the concentration of oxygen in the engine exhaust. The fuel pressure is increased or decreased on a continuous basis to maintain the oxygen concentration in the engine exhaust at a selected level corresponding to a desired chemical composition of pollutants in the exhaust gases. Continuously variable control of the fuel pressure requires the use of relatively costly stepper motor valves, and may also require modification of fuel line plumbing, modification of air induction components, and the addition of an external air or gas supply. Therefore, there is a need for a simple, inexpensive, yet convenient and reliable technique for controlling the air/fuel ratio in a large, industrial internal combustion engine. The present invention satisfies this need.